Method of activating nickel-based catalysts

ABSTRACT

A method for activating nickel-based catalysts and the products resulting therefrom, which comprises annealing and hardening a nickel-based alloy prior to mechanical shaping, mechanically shaping said alloy, annealing and hardening said alloy a second time, isothermally annealing said alloy and thereafter cooling said alloy to about room temperature.

United States Patent Goue et a1.

[ 1 Feb. 29,1972

[54] METHOD OF ACTIVATING NICKEL- BASED CATALYSTS [72] Inventors: Bernard Goue, Gif-sur-Yvette; Louis I Guitard, Paris; Claude Edon, Fontenayaux-Roses, all of France [73] Assignee: Compagnie Generale dElectrlcite, Paris,

France [22] Filed: May 29, 1968 [21] Appl. No.: 733,076

[30] Foreign Application Priority Data May 29, 1967 France ..108159 [52] US. Cl ..l48/2, 75/170, 148/3,

I 148/115, 148/13 [51] Int. Cl ..CZld l/00, C22f 1/10 [58] Field ol'Search ..148/2,3, 11.5, 13; 75/170 [56] I References Cited UNlTED STATES PATENTS Fuller Raney ..75/170 X 1,941,368 12/1933 Smith Primary ExaminerHyland Bizot Assistant Examiner-G. K. White Attorney-Sughrue, Rothwell, Mion, Zinn & MacPeak 1 [5 7] ABSTRACT A method for activating nickel-based catalysts and the products resulting therefrom, which comprises annealing and hardening a nickel-based alloy prior to mechanical shaping, mechanically shaping said alloy, annealing and hardening said alloy a second time, isothermally annealing said alloy and thereafter cooling said alloy to about room temperature.

15 Claims, 3 Drawing Figures FMENTEBFEMQ :972

SHEET 1 OF 2 1/ SUN 5 METHOD OF ACTIVATING NICKEL-BASED CATALYSTS BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates generally to nickel catalysts and more particularly to methods for activating nickel-based alloys.

2. Description of the Prior Art It is known that nickel catalysts can play an essential role in the conversion of chemical energy into electrical energy, in fuel cells.

The principle of such a cell is relatively simple: it is a matter of ionizing by means of appropriate electrodes, the hydrogenated fuel (in the form of I'I ions), and, the combustion support (in the form of ions), and then permitting these two elements to be recombined simultaneously liberating electrical energy.

This invention relates to a method for preparing and activating the catalytic properties of certain nickel-base binary alloys. These alloys are especially useful as electrodes with hydrogenated fuel for fuel cells. Further, the products of this invention can be utilized generally as catalysts for hydrogenation reactions.

SUMMARY OF THE INVENTION In accordance with the method involved in this invention, a catalyst comprising a binary supersaturated solution of a metal, such as beryllium or aluminum, in nickel is activated for fuel cell and other applications. The particular alloy selected for this invention is based upon the following conditions:

The solubility of the second metal in the nickel must vary as a function of temperature;

The second metal must form with nickel an intermetallic compound or an intermediate phase;

The proportion of the second metal in the resulting alloy must be between a lower limit corresponding to the maximum content leading to a single solid solution, at ambient temperature and an upper limit, corresponding to the intermetallic compound or the intermediate phase mentioned above.

This binary alloy is activated by a series of mechanical treatments which are performed while the alloy is being shaped into the desired configuration for its ultimate end use. Specifically, the alloy is subjected to annealing and hardening, shaping to the desired configuration, annealing and hardening a second time, isothermal annealing and finally cooling.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a diagram of a melting furnace according to the invention;

FIG. 2 shows the results of activation treatment applied to a sample of nickel-beryllium subjected to tempering at the temperature of 450 C.;

FIG. 3 shows the results of activation treatment applied to a sample of nickel-aluminum subjected to tempering at a temperature of 600 C.

DETAILED DESCRIPTION OF THE INVENTION In order to clarify the explanations pertaining to the method involved in this invention, it is necessary to review some of the special aspects of catalytic reactions and, particularly, of heterogeneous catalysis, as applied to electrochemistry,

One condition necessary for catalytic activity of the type involved here is the capability of the catalyst to chemically absorb one of the reagents, which form an intermediate compound on the surface of the catalyst corresponding to the reagent and the catalyst. These intermediate compounds then give rise to ions in the solution.

This is the case particularly when we consider a nickel electrode, fed by hydrogen, the latter being absorbed on the metal.

This special feature can be used in order to catalyze, for example, one or the other of the two inverse oxygen or reduction reactions, by varying the conditions under which the reaction is to take place. When we stimulate the reaction toward reduction, molecular hydrogen is generated. On the other hand, we stimulate the reaction toward oxidation, ionized oxygen is generated.

The forces which now originate between the metal and the adsorbate depend on the extent of the metal-metal bonds withinthe mass, which depends very closely on the structure of the metal. Any modification in the structure must therefore, a priori, involve a change in the forces in question, in other words, in the catalytic activity of the metal.

The method involved in this invention is precisely intended, after the preparation of the primary alloy as such, to, modify the structure of the electrodes which are drawn from the alloy during the course of the shaping. These modifications occur in such a way that the catalytic activity of these electrodes is increased.

In shaping the alloy for use as an electrode, the alloy is subjected to a variety of heat treatments. These heat treatments necessarily have a direct effect on the catalytic properties of the metal.

It has been known that the catalytic activity of nickel can be improved through cold-working or strain-hardening. 0n the other hand, annealing tends to adversely affect the catalytic properties of the metal.

It was thus necessary to attribute catalytic properties to the active centers within the metal and, especially, to those in the immediate vicinity of the surface caused by the groups of atoms which were displaced from their equilibrium position. Annealing would remove these active centers, thus causing the metal to lose its catalytic properties.

The method involved in this invention is thus intended to make the electronic factors of the bodies present more favorable.

The metals which catalyze hydrogenation are so-called transition metals whose basic feature is that they have removable d" orbitals. We know that it is possible to describe the activity of a catalyst, for example, an electrode of the type visualized in this invention, in terms of the activation energy with respect to the conversion of parahydrogen into orthohydrogen. We also know that the electrocatalytic activity may be characterized by the intensity corresponding to the ionization of hydrogen in a solution of potash at a fixed potential, with respect to a reference electrode. Finally, we know that the appearance of structural defects and especially the appearance of defects intentionally introduced to activate the electrode can also be detected and its evolution can be made noticeable by measuring the hardness of the alloy obtained.

One means used in the present method to activate nickel consists in displacing the atoms situated in the immediate vicinity of the surface of the metal, while they are in their equilibrium position; the displacements in question can be obtained from the interior of the metal.

This type of displacement causes favorable modifications in the electronic coupling conditions which occur between the activating gas molecules and the metallic atoms on the surface thereby increasing the catalytic and electrocatalytic activities of the metal.

According to another feature of the invention, the abovementioned structural modification must, if it is to be favorable, take place during heat treatment consisting in raising the temperature; this is done so that the resultant activation can be maintained during the heat treatments which will have to be undertaken afterward in order to prepare the electrodes.

According to the invention, one such favorable treatment consists in precipitating or mixture separating, starting with a solid binary solution which is supersaturated and very rich in nickel.

The precipitation is characterized by the presence of a relatively small quantity of precipitate which is-very highly enriched in solute, within a matrix that is much less rich.

The term mixture separation" is used herein as denoting the presence of a large quantity of a precipitate which is relatively poor in terms of solute, as compared to the matrix. Another characteristic of mixture separation is that the matrix and the precipitate have a crystal lattice which are generally of the same nature and whose parameters differ only slightly.

By way of nonrestictive examples for these two structural families, can be mentioned nickel-beryllium alloys to show the precipitation method and nickel-aluminum alloys to show mixture separation.

In each case, the composition of binary alloys is kept between the limits indicated above.

To get a binary solution which will be very rich in nickel, the metals are fused preferably in a vacuum. The alloy so formed is mechanically worked and hardened.

Depending upon the overall composition selected for the alloy in question, the temperature is either higher than the critical solubility temperature of the second metal or is not greater than that of the solidus point.

The alloy is kept at the temperature selected during the time required for the maximum solution of the second metal in the nickel.

On the basis of this temperature, the metal is hardened at sufficient speeds and ambient temperatures, precipitated or mixture separated by means of suitable tempering which is performed at temperatures which are lower than the critical solutility temperatures.

The alloy is hardened during the tempering operations which are normally referred to as structural hardening."

The instability of the solution after hardening gives rise to intermediate structures due to the effect of tempering at low and medium temperatures. These structures are metastable and are increasingly closer to equilibrium as the tempering is accomplished at higher temperatures or as, the duration of isothermal tempering is extended, because diffusion is promoted which leads to precipitation or mixture separation.

The precipitate obtained during equilibrium most frequently is not in any matrix orientation relationship.

In the particular case of nickel-beryllium alloys, an intermediate nickel-beryllium (Ni-Be) phase which is cubic and centered and which is inserted in the primary solid solution is formed on mixture separation.

In the case of nickel-aluminum alloys, the precipitation of a nickel-aluminum phase (Ni -Al) is face centered body cubic.

. The tempering process is tied in with the germination process. After tempering, which follows hardening, structures which are in a higher energy state than equilibrium because of the dispersion in the impoverished solution of precipitates or heterogeneous elements with weak interfacial energy, is obtained. That is to say, they are totally or partially coherent with the matrix.

In the first stage of structural evolution, there is created a series of heterogeneous zones which are closely coherent with the matrix and which are enriched with solute. The disturbances which result from this on the matrix is more or less considerable, depending on the difference in atomic diameters between the solute and the matrix. In the case of nickel-beryllium alloy, it is the beryllium which accumulates in a monoatomic layer, parallel to the planes (100) of the matrix. Because of the considerable difference between the atomic diameters of the two metals present and, also, as a result of the anisotrophy of the elastic constants of nickel, the formation of these heterogeneous zones creates great tensions in the mass.

In the case of nickel-aluminum alloys, a preprecipitation state precedes the mixture separation.

Through the various hardening and tempering treatments performed according to the invention, it is thus possible to produce structural defects ranging from point-shaped defects, caused by the atoms of solute or by the latencies in the supersaturated solid solution, all the way to rather large defects constituted by precipitates that appear at the moment of equilibrium: zones of preprecipitation, coherent with the matrix, or intermediate precipitates, only intermediate precipitates, only partially coherent.

Each of these artificially produced defects in its own way modifies the structure of the material, causing more or less major displacements of the nickel atoms with respect to their equilibrium positions; this consequently leads to variations in the catalytic or electrocatalytic behavior of the electrode.

It can be said that the catalytic activity of a metal, such as nickel, may be attributed to the fact that, since the atoms are displaced from their equilibrium position, a certain number of defects in the structure of this metal are obtained. On this basis we decided, in the method involved in this invention, to perfect special precipitation and mixture separation treatments to be applied to oversaturated binary solutions that are very rich in nickel, in order to create such defects.

We have now defined the broad outlines of our purpose in developing the method according to this invention as well as the means to be employed; we will now describe in detail the application of this method to the preparation of nickel-beryllium and nickel-aluminum alloys; these two cases are given here merely for illustrative purposes and they are by no means restrictive in any way.

In the following, the description of the successive operations will be illustrated for two samples treated, with particular data pertaining to the nature of the second metal being emphasized.

In the two cases, we start with metal having sufficient purity, for example 99.9 percent nickel, 99.97 percent beryllium, and 99.99 percent aluminum. 7

To avoid any accidental contamination, we first clean the various equipment and the raw materials used by chemical methods.

Thus the quartz tubes constituting the interiors of the various furnaces used will be cleaned by means of a baththat might have the following composition:

Hydrofluoric acid, 50% 40 cm." Fuming nitric acid, 48 Be 60 cm. Acetic acid, I00% 40 cm. Bromine a few drops This cleaning is followed by abundant rinsing with deionized water.

The nickel, preferably in the form of fine balls, is degreased, for example, in acetone, and then is dried and deoxidized, for example, in a 50 percent solution of hydrochloric acid, brought to a temperature of 50 C. The balls are subjected for a short time (4-6 seconds), to the action of a bath of 50 percent fuming nitric acid, brought to a temperature of 70 C.; this is followed by abundant rinsing, still using deionized water. Finally, the balls are dried in argon.

As far as aluminum is concerned, it is only degreased with acetone, and then subjected for a few seconds to the action of a 10 percent soda solution.

Prior fusion in a vacuum is performed in a furnace such as shown in FIG. 1.

In FIG. 1, the crucible is made of very pure recrystallized alumina capable of withstanding temperatures of up to l,950 while still suitably resisting heat shocks. The walls of the crucible are quite thin, for example, for a content of about 50 cm., the weight of the crucible is about 300 g. The crucible is placed onto a plate 2 likewiseimade of alumina which is insulated from the crucible by the quartz support 3.

A tube 4 consisting of transparent quartz and not containing any boron, concentrically surrounds the crucible and the plate.

A guide sleeve 5 likewise made of alumina, is fitted to hold the crucible l.

The winding coils 6 provide high-frequency heating energy.

The other parts of the furnace have not been shown because they are all known (the cooled head, the upper portion of tube 4, the thermocouple, the neutral gas, circulation device, the pumping unit, which can generate pressures on the order of 5X10 mm. Hg, the secondary silicone oil diffusion pump, the vacuum measurement gauge, etc.).

The nickel balls, which have been degreased and deoxidized as indicated above, are placed into crucible 1. The second metal is then introduced.

For beryllium, it will suffice to put it between the nickel balls since there is no risk of having the beryllium evaporate prior to the formation of the alloy because its fusion temperature is 1,200 C. and its vapor pressure is only mm. Hg at l,567 C.

On the other hand. for aluminum, which melts at 658 C. and which has a vapor pressure of IO mm. Hg at l,l88'C., it is desirable to incorporate it into the nickel only after the latter is melted. Aluminum is then introduced in the form of a degreased and deoxidized bar, as described above and this bar is progressively plunged into the melting bath.

A vacuum of up to about 7X10 mm. Hg is created and the I high-frequency heating system is started taking care to achieve a progressive temperature rise. Local overheating which might cause the crucible to burst is avoided and the operation is controlled with the degassing gauge.

A slide valve, not shown, makes it possible to insulate the furnace from the beginning of fusion.

Then, argon is slowly introduced until a static pressure of about l00 g. above the atmosphere is reached. The evaporation of the components can thereby be limited.

When using aluminum, the bar is not introduced into the bath until the complete melting of the nickel which takes about minutes before the nickel-aluminum bath reaches sufficient homogeneity. A i W 7 The melt is then solidified gradually by increasing the flow rate of the cooling gas up to about 60 liters per hour. The chamber is again placed under a secondary vacuum until the cooling is complete.

The operation is repeated two or three times so as to eliminate the shrinkage holes and the porosities which the ingot might reveal. WWW H Homogenization annealing is then performed still under a. vacuum and for about 100 hours at l,l0O C. for the nickelberyllium alloys and l,300 C. for the nickel-aluminum alloys but only after the 'degreasing and drying operations as described above. V V HAMVVWWMA As before, the heating device'is started up only when the vacuum reaches a value of about 7X10 mm. Hg.

The next operation involves hardening which must be performed in a minimum amount of time (not to exceed 34 seconds). For this purpose, the piece is placed in a baffle which is immediately subjected to a water shower under pressure, at ambient temperature. 7 g

The piece then becomes sufficiently malleable so as make the first mechanical transformations possible (cutting, laminating, wire-drawing) which is done cold. The plates or wires in the desired dimensions are thus prepared.

The semifinished products which have undergone mechanical transformation are then removed or the samples prior to annealing are annealed by means of dissolving annealing. These operations likewise take place in a vacuum for periods of 2-5 hours, at temperatures between 1,100 and l,350 C.

After the so-called dissolving annealing, the piece is again hardened with water at ambient temperature.

The last operation and by far the most important, is isothermal annealing. p v V V V The conditions must be known here with great precision and this applies to the temperature, which for example, is measured with the help of a Pt/PtRh thermocouple. The exact duration of annealing must also be known. The annealing is terminated by air cooling of the nickel-beryllium alloys and by' water showering the nickel-aluminum alloys.

FIGS. 2 and 3 summarize the results obtained, respectively, 7

FIG. 2 have been plotted, The first two, each characterize the evolution of the structure of the material as a function of the duration of isothermal annealing at 450 C. on the nickelberyllium sample as described above.

The other two curves in the HO. 2 express the practical results obtained, using the alloy in question as a catalyst in two different reaction tests: the electrochemical release of hydrogen and paraorthohydrogen conversion.

The structural characteristics, whose evolution is indicated by the first two curves are the electrical resistance (curve R) and the Vickers hardness (curve D).

The resistance and hardness of each sample was measured by known methods.

As far as the intensity corresponding to the electrochemical release of hydrogen is concerned, its variations are indicated as a function of the linear growth of the electric voltage applied to the sample, with respect to a reference electrode. In the sample shown here, the intensity selected is the intensity which corresponds to a polarization of 920 mv., with respect to an l-lg HgO electrode and for a scanning speed of l v./minute. This characteristic furthermore expresses a rather qualitative aspect'of the evolution of the electrocatalytic activity of the alloy studiedin terms of time.

The paraorthohydrogen conversion reaction test was studied under a dynamic regime. The hydrogen, contained in a bottle, is expanded by a double-cutoff manometer. A needle valve keeps its flow rate constant. The gas is conducted over a bed of activated charcoal which is kept at the temperature of liquid nitrogen. As a result of this passage, the proportion in the mixture of each of the two paraand orthohydrogen isomers is fixed at about 50 percent.

The mixture is then conducted into the first branch of a twobranch conductivity cell; these two branches constitute the two arms of a Wheat-stone bridge. Due to the effect of the catalyst, the proportion of the two isomers in the mixture is modified and this leads to a corresponding modification of the conductivity of the gas mixture passing into the second branch, which throws the bridge out of equilibrium.

The variations thus obtained are continuously registered and the composition of the mixture leaving the apparatus can be determined at any point. This enables the calculation of the reaction speed. From this is derived the activation energy.

The two magnitudes R and D enable, to different degrees, the determination of the direction of evolution of the structure of the alloy being treated, in terms of time.

As far as the intensity of hydrogen release is concerned, it varies in a direction opposite to the activation energy, thereby indicating, the fluctuations in the latter which, as all of the experiments have shown, goes through a more or less regular $9992. 2? ai m m a ums In the diagram, for the particular sample considered, the intensity reveals two maximums, respectively, around the 50th and the 650th minutes. The activation energy reveals two minimums, the first one at about the 50th minute and the second one atthe 450th minute, roughly. m n

The activation maximums, respectively, occur at approximately the 30th and the th minutes. It should be noted that the duration maximum is reached only at about the l,l00th minute, that is to say, quite a bit after the second activation maximum.

'A'Eifih number of parameters are thereby available to enable the characterization of the activation state which the sample achieves during each treatment phase. This is the essential result deriving from the present invention.

in FIG. 3, which pertains to the above-mentioned sample of nickel-aluminum, we have shown the only two results concerning the variations in the Vickers hardness and the intensity of hydrogen release. I it can be seen that the latter presents four maximums, the last of which is the biggest; it corresponds to a treatment dura- 'tion of about 100 hours.

While the invention has been particularly shown and 9 9 2 e e ee mirfi leiafi nt tt t will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

What is claimed is: 1. A method for preparing an activated nickel-based binary catalyst which comprises:

fusing nickel with from about 1 percent to-about 9 percent by weight of a second metal selected from the group consisting of aluminum and beryllium, said second metal being present in an amount of at least the maximum quantity which will form a unique solid solution in nickel, and an amount not more than the quantity corresponding to an intermediate phaseof said second metal and nickel, homogenization annealing and hardening said catalyst in a first operation, shaping said catalyst, annealing said catalyst at a temperature of about l,l C. to about 1,350 C. and hardening said catalyst in a second operation, isothermally annealing said catalyst, and cooling said catalyst to at least room temperature. 2. The method of claim 1 wherein when said second metal is beryllium, said cooling is accomplished in air and when said second metal is aluminum, said cooling is accomplished in 6. The method of claim 1 wherein saidmetals are prog es a 3. The method of claim 1 wherein said fusionis accom;

tion is homogenization annealing and is performed at temperatures of from about l,l00-l,300 C. for approximately hours.

9. The method of claim 1 wherein said first hardening step is performed at ambient temperatures.

10. The method of claim 1 wherein said second annealing procedure is a dissolving annealing operation wherein the alloy is annealed for about 2 to 5 hours.

11. The method of claim I wherein said second hardening procedure is performed at ambient temperatures.

12. The method of claim 1 wherein said isothermal annealing is performed at a temperature of between 450 C. and 600 13. The method of claim 12 wherein said second metal is beryllium said temperature of isothermal annealing is 450 C. and when said second metal is aluminum, said temperature of isothermal annealing is 600 C;

14. The method of claim 5 wherein when said second metal is beryllium, it is introduced into said furnace prior to heating and when said second metal is aluminum it is introduced into said.ur ac a erth n cke is melted. W

15. The method of claim 14 wherein said metals are heated for about 15 minutes to provide said fusion, and wherein at the end of 15 minutes, the heating is reduced and a cooling gas is introduced into said heating zone at a rate of about 60 liters 

2. The method of claim 1 wherein when said second metal is beryllium, said cooling is accomplished in air and when said second metal is aluminum, said cooling is accomplished in water.
 3. The method of claim 1 wherein said fusion is accomplished in vacuo, said nickel and said second metal being previously degreased, dried and deoxidized.
 4. The method of claim 1 wherein said metals are fused in a furnace heated by a high-frequency device.
 5. The method of claim 4 wherein said metals, placed into said furnace, are in the shape of balls.
 6. The method of claim 1 wherein said metals are progressively heated to the fusion point at the rate of 20* C. per minute.
 7. The method of claim 1 wherein an inert gas in introduced into said furnace at a static pressure of about 100 g. above atmospheric.
 8. The method of claim 1 wherein said first annealing operation is homogenization annealing and is performed at temperatures of from about 1,100*-1,300* C. for approximately 100 hours.
 9. The method of claim 1 wherein said first hardening step is performed at ambient temperatures.
 10. The method of claim 1 wherein said second annealing procedure is a dissolving annealing operation wherein the alloy is annealed for about 2 to 5 hours.
 11. The method of claim 1 wherein said second hardening procedure is performed at ambient temperatures.
 12. The method of claim 1 wherein said isothermal annealing is performed At a temperature of between 450* C. and 600* C.
 13. The method of claim 12 wherein said second metal is beryllium said temperature of isothermal annealing is 450* C. and when said second metal is aluminum, said temperature of isothermal annealing is 600* C.
 14. The method of claim 5 wherein when said second metal is beryllium, it is introduced into said furnace prior to heating and when said second metal is aluminum it is introduced into said furnace after the nickel is melted.
 15. The method of claim 14 wherein said metals are heated for about 15 minutes to provide said fusion, and wherein at the end of 15 minutes, the heating is reduced and a cooling gas is introduced into said heating zone at a rate of about 60 liters per hour. 